Μι has no meaning so far other than representing the letter μ. Μαμά is a real word that means mother. It's really confusing trying to figure out when they want a letter name and when they want a word, let alone in the same exercise.

This happens with other letters as well and Duo is aware of this. I reported this for another question and the reply was that they are doing an overhaul of this so-called ABC skill lesson. Don't know when the new lessons are going to come out though.

BTW, you can report it here and someone will reply. But as it is, it's moot.